In this study, the dynamic behaviour of C18 HPLC columns is determined by using the stimulus-response technique. Acetonitrile was used as a non-interacting tracer through the C18 HPLC columns (MikroPak(R) and ELSIsphere(R)). The system was pulse stimulated with 10 mu l of acetonitrile at the flow rates of 1, 2, 3 and 4 ml/min of HPLC grade water. The resulting chromatograms so called response ''C'' curve is obtained for both columns, separately. The kinetics of that tracer were evaluated in a model study. The model equation was solved using parameter estimation by cybernetic moment technique'' and than Peclet numbers, which is the basic indication of the dynamic flow characteristics of the columns, and axial dispersion coefficients, were calculated.